Pumps for pumping molten metal are used in furnaces in the production of metal articles. Common functions of pumps are circulation of molten metal in the furnace or transfer of molten metal to remote locations along transfer conduits or risers that extend from a base of the pump to the remote location. The pump may be located in a separate, smaller chamber such as a pump well adjacent the main hearth.
Currently, many metal die casting facilities employ a main hearth containing most of the molten metal. Solid bars of metal may be periodically melted in the main hearth. Metal scrap such as from aluminum cans is often charged into the molten metal in a scrap well adjacent the main hearth. A transfer pump is located in a separate well adjacent the main hearth. The transfer pump draws molten metal from the well in which it resides and transfers it into a ladle from which the molten metal is taken to a holding furnace and fed into a plurality of die casters that form metal articles. Die casting furnaces employ only a transfer pump, not a circulation pump. When scrap metal is added, it lowers the temperature of the molten metal. Burners located above the molten metal in the main hearth must maintain molten metal temperature while compensating for the drop in temperature caused by scrap charging. A tremendous amount of fuel is required by the burners to heat and maintain the molten metal at a suitable temperature.
In view of the heat applied by the burners at the surface of the molten metal and the cold scrap added to the bath, temperature differences arise in the bath. For example, in a die casting furnace the temperature of molten aluminum might be 1550° F. near the surface of the bath, 1250° F. in a location where the scrap is charged, and 1350° F. near the bottom of the bath. Important considerations in a die casting facility include the consumption of fuel and cleanliness and physical properties of the cast metal articles. Aluminum oxide is formed on the surface of the molten metal as the molten aluminum oxidizes. Aluminum oxide has an affinity for hydrogen gas. It is undesirable to have hydrogen gas in the metal. As the cast metal solidifies it releases trapped hydrogen gas, forming pin holes in the metal articles. Higher temperatures of molten aluminum lead to increased absorption of hydrogen gas and increased pin hole defects with resulting compromise in the physical properties of the metal articles.
Various devices have been proposed for use in submerging metal scrap. One such device disclosed in U.S. Pat. No. 6,217,823 includes a ramp located in a charge well for creating a vortex that pulls scrap down into the molten metal. A drawback of this scrap submerging system is that all of the molten metal is required to be passed through the scrap well. This poses pumping inefficiencies during times when no scrap is being charged. In addition, this may lead to increased generation of oxides due to the greater surface area and turbulence involved in passing molten metal along a vortex all of the time.